The present invention relates to a differential valve for a sub-sea flexible pipe and, more specifically, relates to a valve with two clacker elements capable of discharging gas, for example contained in a pressurized fluid flowing through the flexible pipe.
Flowing through flexible pipes used, in particular, in off-shore oil drilling, is a pressurized fluid which may contain gas which has to be discharged into the ambient surroundings. This gas generally diffuses into an annulus of the flexible pipe. To discharge the gas, it is usual for a drainage valve to be mounted at least at one of the ends, with one side of the valve communicating with at least the annulus whereas the other side communicates with the ambient surroundings which may be the atmosphere when the valve is mounted at that end of the pipe which is above the water surface, or seawater when the said valve is mounted near to the submerged end of the flexible pipe.
Such valves are intended to open as soon as the internal pressure obtaining in the annulus exceeds the pressure obtaining on the outside of the said flexible pipe by a certain amount, it being possible for this pressure difference to vary between 1 and 4 bar for example.
Among the valves used in sub-sea flexible pipes, mention may be made of valves comprising a two-part valve body into the bottom of which there open two passage ducts, a clacker element urged towards the closed position by a preloaded spring, a relatively broad and thick seal set into the lower end of the clacker element, and a flexible diaphragm in the shape of an annulus, the exterior periphery of which is trapped between the two parts of the valve body, and the interior periphery of which is trapped between two parts which make up the body of the clacker element. Two chambers, an upper one and a lower one, are therefore formed in the valve body, the said chambers being isolated from one another by the flexible and deformable diaphragm. The upper chamber of the valve is constantly filled with the ambient surroundings, air or water, which may or may not be laden with impurities in suspension.
When the impurities are not corrosive or not abrasive, the constituent elements are not attacked to any great extent and do not seize up too quickly. By contrast, when these impurities consist of sand, which is particularly the case when the valve is near the sea bed or the shoreline, then premature wear of the said constituent elements occurs, and some of these might even seize up, this sometimes causing the valve to stop working.
Another drawback of this type of valve lies in the fact that it is necessary to match the elements which have to cooperate with each other. This is because each of the constituent elements is manufactured with a certain tolerance. When two elements are matched it is essential that they be matched with the same manufacturing tolerance. Such matching is expensive because it necessarily means that there are elements which are not used.